The present invention relates to material handling vehicles, and more specifically to control systems for determining a steer center position and controlling steering correction for a material handling vehicle which is automatically guided along a wire.
In recent years, changing manufacturing and retail distribution practices have led to significant changes in warehousing practices. Just-in-time manufacturing, for example, makes it necessary for industrial distribution centers to be able to access small quantities of specific materials quickly. Likewise, the advent of on-line and internet shopping has made it increasingly important for retailers to be able to quickly and efficiently locate and retrieve specific items for a number of individual orders. In conjunction with these trends, it has become necessary to store both a greater variety and a greater quantity of goods at each individual warehouse.
To accommodate these changes, automated guided vehicles have become increasingly common. Automated guided vehicles (or AGVs) are vehicles which are guided automatically along an established path such as along a wire in the floor of a warehouse. AGV""s can be unmanned vehicles for carrying parts or components in a manufacturing plant or manned vehicles which carry an operator and provide material handling functions such as order picking, lifting, or reaching. When used in conjunction with a central processing system, the storage and retrieval of specific items can be quickly and easily controlled from a central location, essentially without the need for human intervention. These devices, therefore, make warehousing operations both more efficient and less expensive than prior art manual systems.
Although AGVs can increase efficiency and lower the cost of warehousing operations, automatic vehicles present a number of problems for users. To prevent damage to the racking and shelving used to store materials, as well as to the vehicle itself, and to prevent damage to stored materials, for example, the AGV must be prevented from veering into racking at the sides of the path, or into stock stored temporarily in locations within an aisle or pathway. It is important, therefore, that AGVs move within a predetermined, closely defined path.
Due to these restrictions, steering control systems are vitally important in AGVs. One important parameter for controlling the motion of an AGV is the identification of a steer center position. The steer center position identifies the angle at which the steer motor or other steering device must be directed to drive the AGV in a straight direction. To a close approximation, the steer center position can be established by switches or other indicating devices mounted to the steer mechanism. However, errors in this position can be introduced by mechanical linkages and gear systems in the drive system of the vehicle, by environmental factors in a warehouse, or simply due to changes in control as the AGV is used. To prevent wandering due to these various errors, prior art systems often included a xe2x80x9clearnxe2x80x9d mode for teaching a steer center position to an AGV. In the learn mode, the control system learns the position of a feedback device (such as an encoder) while the AGV is driven along a straight path. While adequate in some applications, however, there are a number of disadvantages associated with the xe2x80x9clearn modexe2x80x9d process. First, additional hardware such as key pads or switches are required to establish a learn mode. Second, once established, the steer center position is not automatically adjusted for varying conditions. Furthermore, the learn mode cannot easily adjust for manufacturing errors related to mechanical linkages and home switches, and therefore can make the manufacture of AGVs difficult.
There remains a need, therefore, for an improved method and apparatus for determining a steer center position for an AGV. Such a method would preferably allow an AGV to automatically learn a steer center position during normal operation, and would automatically modify the steer center position as environmental and mechanical conditions change. The method would also account for errors induced by mechanical linkages, environmental conditions, and general usage of the AGV.
The present invention is a method and apparatus for determining a steer center position for an automatic guided vehicle which follows a wire. As the vehicle moves along a guide wire, the distance of the vehicle from the wire is monitored. At a predetermined maximum travel distance, the average distance from wire over the distance traveled is calculated, and the average distance from wire is used to determine an adjustment to the steer center position. Adjustment to the steer center position, therefore, relies almost entirely on a single variable: the average distance from wire.
The method of the present invention can be used to allow an AGV, and particularly an order picker forklift, or other manned material handling vehicle, to xe2x80x9clearnxe2x80x9d a steer center position, where an initial steer center position, determined by crossing home switches or other known means, is adjusted by a steer center adjustment value to account for differences in the AGV topology due to manufacturing tolerances, mechanical linkages, differences in steer center position in forward and backward motion, misalignment in the construction of the chassis and other factors. Furthermore, the method can be used to adapt the steer center position for changes induced during use. Such changes can occur, for example, due to wear on mechanical parts, adjustments made during maintenance or service of equipment, warehouse environmental factors, damage to home centering switches or switch-activators, or other factors. The present invention eliminates the need for reliance on accurate home switches and specialized xe2x80x9clearnxe2x80x9d modes which must be run to establish a steer center position. The present system can also be used to adapt the steer center position during operation, and therefore to maintain the vehicle within a predetermined distance of a predetermined path, particularly a guide wire path.